Why don't "just" assign enough RAM to your virtual machines instead, and omit the swap partitions alltogether? Overcommiting (backed by the host's swap) and KSM might even help more, than using zram for swap.

swap on a ramdisk (compressed or not) doesn't really sound like a good idea, according (commercial) attempts on windows have been debunked as expensive hoax already. There certainly is a use case for google, but it's a pretty narrow one.